Automatic transmissions to be mounted on vehicles such as automobiles include planetary gear sets and a plurality of frictional engagement elements such as multiplate clutches and multiplate brakes. The plurality of frictional engagement elements are selectively engaged depending on an operation state of an engine so that gears are automatically shifted to predetermined gear positions. The frictional engagement element includes a plurality of friction plates arranged with clearances therebetween, and a piston configured to press the friction plates. The piston moves between an engaging position where the friction plates are brought into an engaged state by pressing the friction plates, and a releasing position where the friction plates are brought into a released state by releasing the pressing.
The frictional engagement element is required to perform engagement with a quick response. Japanese Unexamined Patent Publication No. 2014-081048 discloses an automatic transmission including clearance adjusting pistons configured to reduce the clearances of the friction plates. By reducing the clearances in advance, the engagement response of the friction plates when an engaging hydraulic pressure is supplied can be made satisfactory.
In the transmission of Japanese Unexamined Patent Publication No. 2014-081048, however, a problem arises in that positional control of the clearance adjusting pistons with a hydraulic pressure is difficult. Specifically, when the pistons receive a hydraulic pressure, the pistons are deformed, which makes it difficult to perform positional control of the pistons in an engaging direction so as to reduce the clearances of the friction plates accurately. In this case, a trouble of an increase in engine resistance occurs if the transmission is an automatic transmission having a configuration in which power of a drive source is applied to an input section without intervention of a hydraulic power transmission device (torque converter).